The present invention relates generally to the field of semiconductor device fabrication. More specifically it relates to the manufacture of Light Emitting Diodes ("LED"s).
LEDs are known. During their manufacture they are typically encapsulated in epoxies, polyesters or similar thermosetting materials to protect them from the environment. This encapsulation has been done on both individual LEDs and groups of LEDs fabricated together to form displays, lamps, and other devices.
During the manufacture of semiconductors, including LEDs, it is common to apply a coating to the die before encapsulating it. The purpose of this coating is to serve as a barrier layer, preventing moisture and other materials from contacting the die and damaging it. These coatings vary from a soft gel to a hard shell with a glass transition temperature ("Tg") of 180.degree. C. or more.
In operation, changes in the temperature that the LED is exposed to place stress on the semiconductor die that comprises the light emitting component of the LED. These stresses are caused by the different expansion rates of the various material which comprise the LED when they are exposed to environmental extremes. These stresses are known to damage the die and to reduce the light output of the LED severely in certain circumstance, particularly when the LED is exposed to extremes of temperature.
Although a great deal of effort has been expended in trying to eliminate or counteract the effects of these stresses on the die, no wholly satisfactory solution has yet been discovered. The coatings used to date have not succeeded in eliminating these problems. With gelled coatings, separation frequently occurred at the interface with the thermoset, causing a light trap which reduces the light output. Other failure modes including bond wire breaks were caused by these coatings.